Treat the Fish Pond as a planning system, not a mystery box. The first question is whether the fish you store would earn more by sitting in the pond than by funding the next upgrade immediately.
Early players should avoid locking too much value in pond experiments. Mid-game players can use the pond as a stable income layer once they know which routes they can repeat.
The important decision is timing. If you still fail dives because of oxygen or weak gear, immediate selling may buy the upgrade that makes every later route safer. If your route is already consistent, Fish Pond testing becomes a way to turn known catches into background value.
Simple rule
If selling the fish buys an upgrade that makes every run safer, sell first. If your route is already stable, test pond income with repeatable fish.